Evaporative cooling systems, which are also sometimes referred to as evaporative humidifiers because they both cool air and humidify air, are increasingly being used for data center cooling, among other applications in air-handling systems within both residential and industrial buildings. It is desirable that these evaporative cooling systems provide precise temperature control with the lowest pressure drop possible so as to minimize fan power consumption.
One such system is the FA6™ evaporative humidifier/cooler manufactured by Munters Corporation. The FA6™ evaporative humidifier/cooler is described in a Munters publication entitled “Evaporative Humidifier/Cooler FA for AHU's Technical Manual” and numbered HC/MMA/TGB-1711-06/10, the disclosure of which is incorporated herein by reference in its entirety. In an evaporative cooling system 100, as shown in FIG. 1, for example, water is supplied to the top of evaporative media 102 by a distribution header 104. The water flows down the surfaces (typically corrugated) of the evaporative media 102, and warm, dry air (input air 112) is directed across the evaporative media 102 to an upstream face 106 of the evaporative media 102. As the warm, dry air (input air 112) passes through the evaporative media 102, it evaporates a portion of the water. The energy used for evaporation is drawn from the input air 112 itself, resulting in cool, humidified air (output air 114) leaving the downstream face of the evaporative media 102.
When wetted, a given evaporative media 102 geometry and air flow rate will provide a set evaporation efficiency. This efficiency can be calculated as the Wet Bulb Depression Efficiency (WBDE) defined as the degree of cooling of the air from dry bulb temperature (DBT) to its wet bulb temperature (WBT) measured in percent efficiency. When the evaporative media 102 is dry, the efficiency is zero, and no cooling is accomplished. It is not practical to have the evaporative media 102 partially wetted as repeated wet/dry cycling leaves behind scale from the water each time it is dried. Repeated wet/dry cycles are also detrimental to media life. When water is first flowed over the evaporative media 102, full cooling is seen within minutes. When water flow is halted, however, the cooling effect remains while the water absorbed in the body of the evaporative media 102 evaporates. This time varies based on a large number of variables including the air velocity, DBT, WBT and media type, as different media geometries and compositions hold differing amounts of absorbed water. Typically, it may take 20 minutes or longer for the evaporative media 102 to completely dry and lose its cooling capability after water flow is terminated.
It is often important to provide cooled or humidified output air 114 at a desired set point. This cannot be accomplished with a simple wetted evaporative media 102 alone. Even ignoring the transition period from dry to wet or wet to dry, either the input air 112 is cooled to the temperature defined by the properties of the input air 112 (e.g., temperature and humidity) and media efficiency, or the input air 112 is not cooled at all. Unless the set point is one of these values, then the output air 114 will not be cooled to the required set point. As a result, various methods and systems have been suggested to control the evaporative cooling system 100 to regulate the temperature and humidity of the output air 114.
The simplest form of control is to arrange the evaporative media 102 in individual banks with separate water distribution systems. Water flow is controlled by pumps or valves to wet only the number of banks required to provide the desired cooling. By mixing the cool air off the wetted media with the warmer air from the non-wetted media, a blend temperature can be achieved close to the desired temperature set point. Since the evaporative media 102 initiates cooling quickly after being wetted, the system responds to the requirement for increased cooling rather quickly, but it is slow to reduce the amount of cooling as the media remains wet for an extended period of time after water flow is terminated (as discussed above). In addition, this method of control provides only discrete changes to the cooling capacity based on turning on or off an individual bank. As a result, this method does not provide continually variable cooling control.
Another way to regulate the temperature and humidity of the output air 114 is through the use of bypass or face and bypass control. An evaporative cooling system 200 using face and bypass control is shown in FIG. 2. In this system 200, a series of individual evaporative units 210, 220, 230, 240 are aligned in a plane with a bypass section 250. Each individual unit 210, 220, 230, 240 (also referred to herein as a cassette, and described in the FA6 manual identified above) includes evaporative media and a water distribution header 212, 222, 232, 242. In the system shown, a portion of the cassettes 220, 230, 240 may be selectively used. A solenoid valve 224, 234, 244 may be opened to allow water to flow to the corresponding distribution header 222, 232, 242 and selectively turn on the evaporative cooling capability of the cassettes 220, 230, 240. A bypass damper 252 is used to open or block, selectively and variably, the bypass section 250 and an optional face damper 214 is used to block or open flow, selectively and variably, to at least one of the cassettes 210.
In the evaporative cooling system 200 using face and bypass control, the temperature and humidity are controlled by varying the distribution of air (input air 262) that flows through the cassettes 210, 220, 230, 240 and through the bypass 250. The warmer, dryer air that flows through the bypass 250 (bypass air 264) is mixed with the cooler, more humid air that flows through the cassettes 210, 220, 230, 240 (conditioned air 266). The desired temperature and humidity of output air 268 can be achieved by varying the ratio of bypass air 262 to the conditioned air 264 (blend ratio). For example, the amount that the face damper 214 and bypass damper 252 are opened is adjusted to achieve the desired blend ratio of the output air 268, and the necessary number of solenoid valves 224, 234, 244 are opened to supply water to the desired cassettes 210, 220, 230, 240.
Face and bypass control does allow continually variable cooling control and has a rapid response to changes in desired temperature. There are several detriments, however, with face and bypass control. The dampers 214, 252 and bypass 250 take up physical cross-sectional space in the air handling system, thus reducing the area available for the evaporative media, and resulting in a higher media face velocity. The increase in velocity will result in an increased pressure drop and thus will create a demand for more fan power to overcome the resistance. The face damper 214 may cause high-velocity channeling when partially open. These higher velocity “jets” may disrupt the water flow in the channels of the evaporative media and can cause water to blow off the downstream face of the evaporative media. For this reason, the face damper 214 is often not included, and the less precise control of only a bypass damper 252 is accepted.
Another deficiency of face and bypass control is stratification of the output air 268. As a result, the output air 268 does not reach the desired temperature condition because the hotter air flowing through the bypass 250 is in a separate layer that is dimensionally separated from the cooler air flowing through the cassettes 210, 220, 230, 240. This deficiency is of great concern in data center cooling, where coolers are closely coupled to server aisles, and there is little opportunity for air blending between the cooling system exit and the server air entrances. Adding multiple bypass 250 sections may help mitigate this detriment; however, this approach adds cost, further reduces the plenum area available for the cooling media, and further increases the fan power requirement.
An adaptation of the face and bypass system that is designed to mitigate the effects of the air “jets” problem noted above, while providing consistent system air pressure drop, is described in U.S. Pat. No. 6,085,834.
Further improved control for evaporative cooling systems, particularly for data center cooling applications, is desired